A light diffusing element is widely used in illumination covers, screens for projection televisions, surface-emitting devices (for example, liquid crystal display devices), and the like. In recent years, the light diffusing element has been used for enhancing the display quality of liquid crystal display devices and the like and for improving viewing angle properties, for example. As the light diffusing element, for example, there is proposed a light diffusing element in which fine particles are dispersed in a matrix such as a resin sheet (see, for example, Patent Literature 1). In such light diffusing element, most of incident light scatters forward (output plane side), whereas apart thereof scatters backward (incident plane side). As a refractive index difference between each of the fine particles and the matrix becomes larger, diffusibility (for example, a haze value) increases. However, if the refractive index difference is large, backscattering increases. More specifically, there is proposed a technology for placing a light diffusing element on the top surface of a liquid crystal display device so as to enhance the display quality of the liquid crystal display device. However, such light diffusing element does not have sufficient light diffusibility (for example, a haze value of less than 90%), and dose not exert any sufficient effect of improving the display quality. On the other hand, in the case where a light diffusing element having large light diffusibility (for example, a haze value of 90% or more) is used in a liquid crystal display device so as to enhance the display quality, when outside light is incident upon the liquid crystal device, a screen becomes whitish, resulting in a problem in that it is difficult to display a video and an image with a high contrast in a bright place. This is because the fine particles in the light diffusing element cause the incident light to scatter backward as well as forward. According to the conventional light diffusing element, as a haze value becomes larger, backscattering increases. Therefore, it is very difficult to satisfy both the increase in light diffusibility and the suppression of backscattering. Further, in an illumination application, as a haze value becomes larger, backscattering increases and a total light transmittance decreases, which degrades light use efficiency.
As means for solving the above-mentioned problems, based on the concept of suppressing the reflection at an interface between each of the fine particles and the matrix, for example, there are proposed: core-shell fine particles, in which the refractive index of a core is different from that of a shell, and fine particles having gradient refractive indices, such as the so-called gradient index (GRIN) fine particles, in which the refractive index changes continuously from the center of each of the fine particles toward the outer side, are dispersed in a resin (see, for example, Patent Literatures 2 to 8). However, with any one of those technologies, a thin light diffusing element with a high haze cannot be obtained. For example, in the GRIN fine particles of Patent Literature 8, when a thickness of a refractive index change portion is defined as L (nm) and a refractive index change amount of the refractive index change portion is defined as Δn, a steep refractive index change portion with a Δn/L of 0.00053 (nm−1) is formed. However, a light diffusing film using the GRIN fine particles of Patent Literature 8 only gains a haze of 86.5% even when a film thickness thereof is set to as large as 20 μm. As described above, there is a strong demand for a thin light diffusing element with a high haze (having excellent light diffusibility).
Meanwhile, along with the expansion of applications of liquid crystal display devices in recent years, various new problems have arisen. For example, in mobile telephones, in order to impart durability and design property, a plastic substrate (in general, an acrylic plate) is placed on a liquid crystal display portion. Further, in in-car displays such as a car navigator, tablet PCs used often industrially, public displays, and multi-functional mobile telephones, a touch panel is placed on the surface of a display portion (see, for example, Patent Literature 9). Such front substrate (for example, a plastic substrate or a touch panel) and a polarizing plate are usually fixed with a double-sided tape attached to an edge portion of the polarizing plate. The thickness of the double-sided tape is generally about 120 μm, and hence, there is a problem in that the thickness of the entire liquid crystal display device increases. In the case of using such double-sided tape for the touch panel, in order to minimize impact, sponge of about 1,000 μm is also used together with the double-sided tape, and hence, the thickness further increases. Further, only edge portions are bonded to each other with the double-sided tape, and hence, an air layer is formed between the polarizing plate and the front substrate. The refractive index of air is about 1.0, whereas the refractive index of a member forming the front substrate such as a polymer or glass is about 1.4 to 1.7. Thus, the following problem arises: a refractive index difference between the air layer and the front substrate becomes large, and hence, visibility in a bright environment is degraded owing to interface reflection of external light. Further, in the liquid crystal display device, a color filter layer of a liquid crystal cell generally functions as a screen. In the case of using a touch panel as the front substrate, an input hitting point is on the surface of the front substrate. In this case, the following problem arises: there is a distance between the surface of the liquid crystal cell serving as a screen and the surface of the front substrate, and hence, parallax is caused.
In order to suppress reflection of external light and glittering of a display screen in a liquid crystal display device using a front substrate, there has been known a liquid crystal display device in which a front substrate and a polarizing plate or a display are attached to each other with a pressure-sensitive adhesive layer having a light diffusing function interposed therebetween (see, for example, Patent Literatures 10 and 11). However, the light diffusing pressure-sensitive adhesive layer needs to be thick in order to realize a high haze (impart light diffusibility), which makes it difficult to realize a reduction in thickness.
Further, in recent years, an attempt to suppress power consumption of the liquid crystal display device has been made. The liquid crystal display device is generally being developed for a panel portion and a backlight portion separately, and the attempt to suppress power consumption has been made mainly in the backlight portion. FIG. 23 illustrates a basic structure of a general direct type backlight unit. Light sources 551 are arranged at a predetermined interval in a lamp house 550 having a reflective film attached to an inner surfaced thereof. A diffusion plate 552 is placed above the lamp house 550 for the purposes of holding the shape of the lamp house, eliminating a lamp image, and the like. In general, it is difficult to eliminate a lamp image only with the diffusion plate 552, and hence, a few diffusing sheets (diffusing films) 570 are placed. Further, for the purpose of enhancing brightness, a brightness enhancing sheet 110 such as a reflection type polarizer is placed. Light emitted from a backlight source is finite, and hence, there is a demand for enhancement of use efficiency of the backlight source. In order to fulfill this demand, for example, there has been proposed a backlight unit using a reflection type polarized light separation element including a metal lattice arranged on a wire grid (see, for example, Patent Literature 12). However, there is a demand for further enhancement of the use efficiency.